Secondary lithium batteries, often known as rechargeable lithium batteries in consumer electronics, have shown great commercial promise due to its relatively high energy density, negligible memory effect and a low loss of charge when not in use.
A typical secondary lithium battery is composed of three primary functional components: a positive electrode (cathode) made from a lithium metal oxide, a negative electrode (anode) made from lithium metal or lithium insertion materials such as lithium alloys or carbon and an electrolyte communicating the two electrodes. Each of the battery electrodes contains an active material (AM) as a primary component, for performing a reversible electrochemical intercalation-deintercalation of lithium ions in the charge/discharge cycles. During charging, an external power source urges lithium ions hosted in the cathode active material to pass through the electrolyte and insert into the anode active material, thus producing a charging current circuit. During discharging, a reversed process is carried out, wherein lithium ions are extracted from the anode and inserted into the cathode.
In order to enable such electrochemical charge/discharge cycles, each electrode in a lithium secondary battery needs to host lithium ions in its active material structure and also possess high conductivity of both lithium ion and electrons. Accordingly, since typical active materials in the electrodes (e.g. lithium metal oxides in the cathode or lithium alloy material in the anode) have low or little electron conductivity, a conductive agent such as a carbon-based nanomaterial must be added together with the low-conductive active material to form a working electrode. Additionally, a binder is also blended in the electrode material, to attach the cathode or anode composite to its respective current collector while providing sufficient cohesion among the powdery active material and conductive agents. A suitable binder material is vinylidene fluoride (VDF) polymer, as is known in the art.
Generally, techniques for manufacturing electrodes involve the use of organic solvents such as N-methyl-2-pyrrolidone (NMP) for dissolving VDF polymer (PVDF) binders and homogenizing them with a powdery electrode material and all other suitable components, including the aforementioned conductive additive, to produce a paste to be applied onto a metal current collector. As a result, the dispersion behaviour of a conductive carbon additive in the organic solvents is an important factor, because carbon particulates having a tendency to flocculate in the solvents are unfavourable for the electrode forming process. Moreover, factors such as surface area and particle size of the conductive additives are found to have strong influence on the charge-discharge capacity and volumetric energy density of the electrode. Among the typically employed conductive additive materials, carbon black is preferably used, due to its satisfactory dispersion stability in the organic solvent, relatively large surface area, and ease of acquisition in the industry. Conductive additives made from carbon black are also much cheaper than those made of carbon nanotubes (CNTs), which are expensive due to low yields during their synthesis and purification process.
However, in the use of carbon black as electrode conductive additives, it has been found that the interaction of this conductive agent with the associated active material in the electrode would notably reduce the number of available charge carrier per volume unit of the electrode, thus leading to an unwanted reduction of “energy density” (in Wh/L) of the battery cell.
Therefore, to improve the energy density of the current lithium battery, it would be beneficial to replace carbon black with a conductive additive having higher conductivity—which could give the battery electrode the required electron conductivity with a smaller weight ratio—for minimizing the energy density loss of the battery active material. In this regard, the carbon nanomaterial of graphene (a single layer and 2-dimensional nanostructure of carbon atoms) was recently found to possess a high surface area to volume ratio and extraordinary electronic transport properties, both properties being superior to the carbon black, making it an attractive conductive additive material in theory. The graphene platelets may be oxidized to various extents during their synthesis, and thus become graphene oxide sheets. Therefore, for the purpose of the present invention, the term “graphene” is used to encompass both pristine, non-oxidized graphene and its oxidized form, and the term “graphene oxide” is intended to denote the oxidized graphene only.
In practice, a main drawback in using pristine graphene material is the difficulty to obtain stable dispersion of pristine graphene in an organic solvent. For example, as reported by Y. Hernandez, etc., when subjecting an initial mixture containing 100 mg/L of exfoliated graphite in NMP—one of the most “efficient” solvents ever reported—to bath sonication, a large number of macroscopic aggregates were observed and the maximum graphene dispensability in the supernatant is merely ˜4.7±1.9 mg/L (HERNANDEZ, Y., et al. High-yield production of graphene by liquid-phase exfoliation of graphite. Nature nanotechnology. 2008, vol. 3, no. 9, p. 563-568. and HERNANDEZ, Y., et al. Measurement of multicomponent solubility parameters for graphene facilitates solvent discovery. Langmuir. 2010, vol. 26, no. 5, p. 3208-13.). The value is much lower than the room-temperature solubility of single-wall CNTs in most organic solvents, as reported by Jeffrey L. Bahr, etc. (BAHR, J. L., et al. Dissolution of small diameter single-wall carbon nanotukbes in organic solvents. Chem. Commun. 2001, p. 193-194.). Therefore, the poor dispersion behaviour of pristine graphene in the polar organic solvent unfortunately imposes a challenge in using it for the electrode forming process.
Despite the dispersion instability issue with graphene material, in recent years research resources have been devoted to developing polymer-graphene composites for electrical applications. For instance, US 2005/0014867 (assigned to WAYNE STATE UNIVERSITY) discloses a method comprising diffusing a polymeric agent in a supercritical fluid between layered particles of a graphite structure, and the method further comprising catastrophically depressurizing the supercritical fluid to form exfoliated graphene platelets surrounded by the polymeric coating agent. Although this route is useful in terms of producing pristine graphene platelets covered with a polymeric coating agent, it nevertheless requires the use of high-pressured supercritical fluid (e.g. high-pressured carbon dioxide gas) and the associated high-pressure sustaining reaction vessels connected with a robust piping and heating system, which renders this production route complicated and costly. Hence, it remains a need to find a relatively simple and easy-to-operate route to prepare a well dispersed graphene-polymer blend in an organic solvent for producing an active electrode.
Therefore, an objective of our invention is to make possible the use of graphene as an effective conductive additive in an electrode, by providing a solvent based electrode-forming composition wherein the graphene is well dispersed in an organic solvent together with at least one VDF polymer, with or without an electrode active material.
It is another objective of our invention to provide a process for producing an electrode-forming composition, wherein the composition comprises at least one VDF polymer and a conductive graphene additive in an organic solvent such as NMP.